I. Field of the Invention
The present invention relates to applications of active electromagnetic shielding to electromagnetic receivers for canceling undesired, interfering signals.
II. Description of the Related Art
It has long been known that voltages are induced in all conductors exposed to changing magnetic fields regardless of the configuration of such conductors. Electromagnetic radiation will induce electrical signals in electronic devices according to the laws of magnetic induction. Thus, it has been desirable in some applications of electronic instrumentation to reduce the inductive noise caused by electromagnetic radiation.
A prior-art method for electronically shielding an electromagnetic pickup from an electromagnetic source is to “unbalance” either the pickup device or the electromagnetic source. Such a method is described by Hoover in U.S. Pat. No. 4,941,388. Hoover provides amplitude adjustment to compensate for amplitude variations between the responses of separate pickups to electromagnetic radiation. However, Hoover does not compensate for differences in the pickup coils that cause the amplitude-variation of the pick-up responses to be frequency-dependent. Thus, Hoover's proposed solution results in poor cancellation over a broad range of frequency. Furthermore, Hoover does not compensate for phase-variations that occur between different pickup coils. The resulting cancellation from the unbalancing method is poor.
Hoover describes the operation of negative feedback in an electrically coupled transceiver. An electromagnetic pickup provides an electrical signal to a drive coil that generates an electromagnetic field to which the pickup responds. Hoover mentions that the system tends to drift from the negative feedback condition at higher frequencies, and identifies the cause of this drift as distortions in the phase-response of the system resulting from the pickup, driver, and amplifier in the system. Hoover does not present an effective method for controlling the phase-response of the system, nor does Hoover present the mathematical relationships between phase and frequency resulting from the driver and pickup coils. Rather, Hoover proposes the use of a low-pass filter to reduce the gain of the system at which the negative feedback condition breaks down.
In U.S. Pat. No. 4,901,015, Pospischil shows a cancellation circuit for canceling the response of a pickup to ambient electromagnetic fields. Pospischil describes first and second pickups that are positioned in parallel with the wave fronts of an interfering electromagnetic field. With such placement, the electromagnetic field impinges simultaneously upon the first and second pickups. The pickups are connected in opposition. Thus, simultaneous impingement of the electromagnetic field upon the pickups is expected to produce a 180-degree phase displacement of the received signals.
If the electrical path lengths of the received signals in Pospischil's cancellation system are different where they are combined (summed), the relative phase difference between the received signals do not have 180-degree phase displacement. Thus, the signals do not completely cancel. In some cases, the signals combine constructively. Pospischil shows that differences in the electrical path length occur when the propagation path lengths of the signals received by the pickups are different (e.g., the signals do not impinge upon the pickups simultaneously). These differences in propagation path lengths can result from reflections, multipath delay, superpositions of multiple received signal components, or received electromagnetic signals having non-perpendicular angles of arrival.
Pospischil does not identify nor compensate for electrical path-length differences (e.g., differences in impedance) that occur between different electromagnetic receivers (pickups). Such pickup assemblies are also used with electric guitars and are known as “hum-bucking” pickups. This technique is not effective for providing a high degree of cancellation because slight differences between the pickups, even pickups that are substantially identical, cause the frequency-dependent amplitude and phase response of the pickups to differ significantly from each other. Thus, the pick-up signals will not be exactly out of phase and equal in amplitude when they are combined.
Coils of wire whose currents support magnetic fields in space function as antennas radiating electromagnetic energy. There are several cancellation methods used with antennas that act as electromagnetic shielding. One of these methods is the basis of operation for a side-lobe canceller that uses an auxiliary antenna in addition to a main antenna. Combining the outputs from the two antennas results in cancellation of the antenna beam pattern in the direction of a noise source so that the effective gain of the antenna in that direction is very small.
Similarly, beam forming in phased arrays provides directional interference cancellation. Beam forming can provide exceptional performance in a fading environment, which is due to the ability of an array to select signals based on the signals directions of arrival. As strong signals are selected for reception, destructive cancellation caused by reflected components arriving at the array elements is mitigated by the placement of nulls. Null placement is also effective in mitigating co-channel interference. However, problems with beam forming include the inability to resolve co-located or closely spaced radio sources unless multipath components of these signals are tracked. In addition, the number of antenna elements limits the number of co-channel interference sources that can be nulled. This is a significant problem because each multipath component arriving at the array is a source of interference. Therefore, a small number of transmitters may provide a large number of interference sources.